Pestiferous tent caterpillars making annual pitch

Editor’s note: Get Growing, written by the La Plata County Extension Office’s Master Gardener Program, appears every other week during the growing season. It features timely tips and suggestions for your garden and landscape.

By Darrin Parmenter

Given the number of phone calls coming into the office over the last few weeks, it seems as if tent caterpillars (Malacosoma species) have made their presence known on our trees and large shrubs this month.

Most often seen on aspens, cottonwoods and Gambel oak, tent caterpillars will also use other trees – alder, crabapple, willows – as host plants. They will also host on numerous shrubs, including chokecherry, mountain mahogany, serviceberry and even wild rose.

The voracious eaters spend their winters as egg masses attached to twigs and branches. Shortly after bud break the newly transformed caterpillars make their way to the crotches of branches and set up camp, which consists of their “tent” (a mass of dense silk). Caterpillars use the tent for shelter and defense, venturing out at night to feed.

While their eating habits may be teenager-like at times, their damage tends to be nothing more than unsightly. The trees typically put out new leaves; however, the additional depletion of carbohydrates directed to the new leaves could stress the tree. Successive years of defoliation may eventually lead to mortality, but in our area we typically see elevated populations lasting no more than a year or two because of natural controls (parasitic insects, viruses, birds, frogs, mice, etc.).

If control is necessary, the combination of removing egg masses (usually after July) and tents by hand may inflict significant mortality to the larval populations. Minimize pruning to avoid injuring the plant. If chemical control is necessary, try using the microbial insecticide Bacillus thuringiensis (trade name: Dipel, Thuricide, etc.). Bt is a naturally occurring bacterium that is common in many soils around the world. Make sure you apply the insecticide late in the day as it does degrade with sunlight, and that you apply it to the leaves as Bt must be eaten to be effective.